1. Field of the Invention
This invention relates in general to monitoring systems for programmable logic controllers (PLCs), and in particular, to a system for developing Man-Machine Interfaces to graphical monitor ladder logic programs executing in PLCs.
2. Description of Related Art
PLCs perform many of the control functions for assembly line machines, machine tools, and other types of industrial equipment. For example, a part arriving at a workstation may contact and close a limit switch or other type of sensing device. As a result, an electrical circuit is completed or broken, and a signal is sent to a PLC indicating the change in condition. The PLC responds to the input as directed by a ladder logic program which, to a large degree, simulates the response of what used to be accomplished by older systems with a set of relays and other control devices. Ladder logic programs instruct the PLC how and when to react to the different signals it receives. The PLC, in turn, instructs the devices it controls in an appropriate manner as specified in the ladder logic program.
In the prior art, operators can communicate with PLCs from personal computers or other control devices to monitor the status of ladder logic programs, troubleshoot ladder logic programs, force the status of I/O status bits ON or OFF to simulate events, and perform a number of other functions. However, prior art systems offer little else in the way of monitoring aids.
For example, in the prior art, an engineer must manually build a monitoring system separately from the development of the ladder logic program. Thus, the engineer constructs a database for the monitoring system, including PLC stations, element addresses, possibly including textual descriptions and other information such as scaling factors and mathematical functions to be performed. Further, keeping these separate entities (i.e., the monitoring systems, the databases, and the ladder logic programs) synchronized can be a logistical nightmare.
Furthermore, in the prior art, an engineer does not have access during monitoring to the ladder logic program. Thus, the engineer typically must exit the monitoring system and separately access the ladder logic program, or use a separate computer for accessing the ladder logic program.